Sweep, Loading… Unloading I

Always shrouded in the mist of mystery, when popular casting mechanics focuses on spey issues it seems to enter the realms of magic.It doesn’t help that the various styles of spey casting seem to compete in presenting their respective approaches as if they were different techniques, instead of just adaptions to some particular conditions.
Fortunately spey is spey, and physics is physics, and the latter governs the phenomena involved in the art of throwing a line with a pole in exactly the same way, whatever the brand, length or taper of your rod and line, and the waters and fish you are after, be it in Scandinavia or in the Pacific North West.

One concept is common to some of those schools, though: that efficiency in spey casting lies, in good part, in some kind of pre-load of the rod prior to the start of the forward cast.

An excerpt from a highly-regarded book will clarify this point:

With the fly/leader anchored to the water surface, the momentum of the forming “V” loop reloads the rod 180 degrees as a reaction.

So when the anchor touches down and the V-loop forms the rod gets automatically loaded. Apparently as the line is traveling backwards it will pull on the rod tip forcing it to bend. Pure logic, isn’t it?

The problem is that physics has the annoying habit of defying what at first sight looks like common sense. The good news is that high speed cameras, and a basic knowledge of Newton laws, help to open a more clear window into reality.
So in order to shed some light on this issue I did set up the following scenario:

Line configuration in front of the caster similar to a perry poke with my right foot stepping on the line tip.

A sweep to set the V-loop.

Without a pause the rod is lifted up to the starting position for the forward cast in a continuous motion, and is stopped there.

Here you are:

An analysis of the casting sequence shown above is in order:

I start the sweep by accelerating the rod butt; I finish the sweep by decelerating the rod butt; as soon as the rod butt speed decreases, rod unloading starts.
Take notice that when I reposition the rod for the forward cast there is no load left in the rod.
After that the V-loop is fully formed and the line gets tight. That tension in the line loads the rod, right? Well, no, as shown by the video above the tight V-loop against the rod doesn’t put a bend on it.
Does it sound strange? Well, in fact basic physics tells us that it couldn’t be any other way. To understand this we have to look at the reason for rod bending, that is, force.

The sweep applies force to the rod, and the rod applies force to the line. Newton taught us that forces always come in pairs, it is what we know as action/reaction. So the rod applies a force to the line and the line reacts applying the same force to the rod in the opposite direction. Flexibility makes the rest.

The caster finishes the sweep by ceasing applying force; no force, no bend; the rod unloads itself. It is capital to take into account that the rod doesn’t need to be completely still to unload, that process happens before: as soon as the caster decreases rod butt velocity the rod unloads. Motion doesn’t necessarily mean force, only accelerated motion does mean force; a decreasing rod butt velocity means that it is not being accelerated anymore, and when acceleration disappears force disappears as well. In summary, a complete stop is not needed for the rod to unload.
At some point in the unbending process the line overtakes the rod tip, and the rod tip pulls on the line forcing it to turn around: a loop is born.

Back to the line in course of crashing against the water: it gets anchored and gets stopped.

What has stopped it? The water (in the case of the video above my foot).

Where does the fly leg momentum go? Obviously to the water, so that force of the crashing anchor is applied to the water, not to the rod!

An old slow motion video showing rod load when anchoring on water:

And another one:

But don’t take my word for it. A very easy experiment for you:

Rig a rod with line and leader. Lay the line in a perry poke configuration like that in the video. Take the end of the leader between your fingers. Make a sweep. Do you feel anything in your rod hand? Do you see any sudden loading? Where do you feel the tug of the line when it gets tight?

[…] it is worth examining rod load in spey casting. So the present article comes as a complement of this one about bend when the anchor settles, and this other one on the effects of circling-up after the […]